Coastal engineering design of a rip-rap revetment ... - Up To - SOPAC

�� Figures 19-22 show large cavities in
various sections of the masonry wall,
produced by wave-induced erosion and
scouring;
�� eroded cavities like those above cause
dislodgment of boulder aggregate from
within the structure leading to loss of
internal support;
�� since masonry walls do not possess tensile
strength, the wall eventually cracks due to
loss of support and integrity and fails;
�� Figure 23 shows erosion and scouring at
the western corner of the rip-rap
revetment system, at the airport fill site;
�� the above is cause by flanking or the
absence of protection at the end of the riprap
revetment system and is the product of
longshore currents;
�� the height of the scarp is about 0.75 m and
exposes coarse, angular limestone fill
material (Figures 24 and 25), with
undercut sections (Figure 25);
�� the entire fill site is contained by a slightly
curved (at the top) concrete gravity wall
(Figure 26), about 6 m above mean sea
level;
�� the top of the wall is about 2.5 m wide;
�� toe protection is provided at the base of
the wall, in the form of 0.75-1.25 m
diameter limestone rip-rap (Figures and 26
27);
�� toe rip-rap may occasionally be dislodged
by large waves and during extreme events
(Figure 27) and therefore not entirely
stable;
�� Figures 28-30 show some features of the
eastern part of the coastal rip-rap
revetment at the airport fill site;
�� from Figure 29 one can assess the extent of
the reclamation and fill project, into the
marine area, is about 175 m at its southern
most point/seaward extremity;
�� the beach on the eastern side of the fill site
is gentle (5°) and is of fine to medium
carbonate sands (Figure 31) with some
small, in-situ karst limestone pinnacles in
the surf zone;
�� the residents on the eastern side of the
airport fill site have also protected their
property with limestone rip rap (Figure
32), indicating erosion problems there as
well.
Coastal Engineering Design, Yaren District, Republic of Nauru
3.2 Recommended Design
SOPAC Preliminary Report 124, October 2000: Russell J. Maharaj
At this time, only a synopsis of a coastal
protection design is presented. Additional and
further details with be presented and discussed in
SOPAC Technical Report 317.
Design information has been produced after
Numerical Analysis with Coastal Engineering
Software CRESS and ACES.
A multi-layered, free-draining rip-rap revetment is
proposed for remediation of the erosion problem
at the site (Table 1). This structure will also
protect the problem area from future erosion by
wave attack under similar hydraulic conditions
discussed in Section 3.0.
The rip-rap revetment should have the following
design elements:
�� two outer layers with a width of 10 m;
this is the primary armor of the
revetment;
�� the revetment should be winged, that is,
the ends of the revetment should not be
open to wave attack, but should be built
into the adjacent land or “closed”;
�� the revetment should utilize natural
dolomitic limestone rock from RON;
�� it should consist of a granular filter layer
or secondary armor layer, made up of
0.20-0.35 m diameter rocks; this underlies
the primary armor;
�� the revetment should have a 1:1.5 seaward
slope;
�� a geotextile filter fabric is also
recommended for use in this structure;
this fabric is a free draining artificial
media;
�� the geotextile fabric should have
perforations with dimensions less that the
diameter of the smallest boulders to which
it is juxtaposed;
�� two separate layers of the geotextile filter
fabric should be used;
�� one layer will underlie the primary armor
of the revetment and overlie the granular
filter media;
�� the second liner should overlie the natural
soil/land and underlie the secondary
armor;
�� the rock revetment should use 0.89 m
nominal size dolomitised limestone
boulders obtained locally (from RON).
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